How to select all textures used by all active materials in the scene?
If you save then reload your file, Blender will get rid of the unlinked (unused) images. (and Materials, by the way)

Then you can use File/External Data/Pack all into file, you should end up with a .blend that contain every image used in the scene.

If you need to keep them as separated files, you can copy your packed .blend in a different folder, then use External Data/Unpack all into files.